Here is how you do it. It's two simple mathematical equations your 4th grader could do.


So you have a P265/75R-15 Tire. What are the real measurements? Well let's figure out what that series of numbers mean first.

P - Passenger vehicle tire (LT is also common LT= Light Truck tire)
265 - Cross section of the tire, not tread width, in millimeters (mm)
75 - Percentage of cross section (75% or .75)
R - Radial tire (as opposed to a bias ply)
15 - Wheel diameter (eg. 15", 16", 16.5")

Now here are the equations-

Tire section (in mm) divided by 25.4 = Tire section in inches. There are 25.4mm in an inch, that's where the 25.4 comes from.

Tire section (in inches) x Percentage of cross section x 2 + rim diameter (eg. 15", 16", 16.5") = Tire diameter
265 / 25.4 = 10.433= Section width
265 / 25.4 x .75 x 2 + 15 = 30.649= Tire Diameter

So, the tire size is 30.649 x 10.433 or about a 31 x 10.50 flotation size

Advertised flotation sizes are really nominal. Tire manufacturers are not required to make a 33 x 12.50 tire actually measure a true 33" tall with a 12.5" section width. Some 33's could measure 32". Interco probably has the best reputation for keeping a true tire size to advertised tire size. Interco also has many calculatiors for gear ratios and tire sizes and so on. Check it out. Try this one and see what you get-

LT285/75R-16

See below for an answer-

































If you cheat you aren't learning anything!











































































Section width = 11.220
Diameter = 32.831
or about a 33 x 11.50 in a flotation size